Can't figure this one out. Have a '11 automatic 4x4 taco. Put it in 4WD went up a small hill let off the gas and rolled backwards down (Still in D) about 50 ft. Looked at my dash and noticed the engine had quit. What happened? Is this bad for the trucks tranny, engine, etc?

Since you rolled backwards in drive you forced the torque convertor to pump tranny fluid backwards so it slowed down the engine side enough to cause it to stall. I wouldn't say it is bad but I wouldn't' make a habit of it.

The offroad models have HAC to prevent stalling.

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The pump is attached to the engine side of the tranny, and only pumps fluid one way, unless the engine is forced to spin backwards. What causes the stalling of the engine, is the Stator is a "one way" design, and if in a forward gear, and the truck pushed in reverse, too much load is created causing the engine to stall.